Multi-pane windows have been widely used as a means for decreasing heat loss in residences and other buildings. Such multi-pane windows usually are constructed by joining two single-pane windows together with a seal made of a polymeric material such as polysulfide, butyl, or polyisobutylene. A metal spacer typically is disposed intermediate the panes about the periphery thereof in order to maintain a desired spacing of the panes. Frequently, the spacer will be coated by a portion of the seal or will be disposed within the seal so as to perform a combined spacing/sealing function.
A problem exists with sealed multi-pane windows in that fogging of the window panes can occur. That is, moisture trapped in the volume defined by the spaced, sealed panes can condense on the panes, thereby fogging the panes and reducing light transmissibility. This type of fogging is known as aqueous fogging because it is caused by the water content of the atmosphere within the sealed volume.
A second type of fogging, known as chemical fogging, relates to the type of material used to seal the window panes. The seal material typically is a high molecular weight polymer that generates, or releases, solvents such as toluene or xylene, or volatile organic oligomers, by-products, degradation products, processing aides, tackifier resins, plasticizers or the like. These so-called off-gases will migrate into the volume defined by the spaced panes; the off-gases can condense on the window panes so as to produce fogging or discoloration of the panes.
Chemical fogging can be particularly acute with low emissivity glass. Low emissivity glass is glass that has been treated on its inner surface with various metal oxides and metals in order to reduce the emissivity of the glass. A typical coating for low emissivity glass includes layers of iridium oxide and elemental silver, as well as an additional layer of iridium oxide (or zinc oxide or titanium oxide).
Various techniques have been used in an attempt to solve the problem of chemical fogging. It has been known to use adsorbents inside the volume defined by the sealed window panes in order to prevent aqueous fogging and chemical fogging. The adsorbents have been mixed directly into the seal, or the adsorbents have been disposed within the spacers (where the spacers are hollow). Adsorbents that have been used include silica gel, carbon black, and various molecular sieves, such as MOLSIV adsorbent, commercially available from the Union Carbide Corporation, Tarrytown, New York 10591. While such adsorbents have been effective in preventing fogging of clear glass, they are generally ineffective in preventing chemical fogging of low emissivity glass.
Desirably, a technique would be available that would prevent chemical fogging of low emissivity glass. The technique desirably would be inexpensive, easy to install, and effective throughout the expected life of the window.